It is conventionally known from the prior art to apply in technical solar cell printing a contact to a substrate of a solar cell by means of a printing screen. In particular it is conventionally known to print a metallization, contact and/or conductor tracks of a contact for a solar cell using printing screens by applying a printing paste which usually contains silver by means of a doctor blade through printed image openings of a printing screen to a substrate of the solar cell, wherein the printed image openings of the printing screen substantially correspond to the printed image or at least to part of the printed image of the solar cell contact to be printed.
Such printing screens have a wire screen mesh which is clamped in a frame, is embedded in a structure layer, such as a thin photoemulsion layer (see a printing screen according to DE 10 2007 052 679 A1, for example), and carries the structure layer and/or acts as a carrier layer for the structure layer. Such a photoemulsion layer has the printed image openings corresponding to the printed image of the contact to be printed, wherein the screen mesh stabilizing the photoemulsion layer also extends in the region of the printed image openings of the structure layer. For the production of such printing screens, the wire screen mesh is usually clamped on a frame and is then coated with a photosensitive material (e.g. an emulsion layer or a film). Then, the printed image is structured by exposing the photosensitive material, for example.
However, the use of such printing screens for applying the contact of the solar cell to the solar cell substrate involves drawbacks, in particular with respect to the printing of what is called the contact fingers of a front contact for the solar cell. The contact fingers shall be printed onto the substrate with the least possible width (e.g. in the range from about 20 μm to 100 μm) at a height as uniform as possible to render possible a conductor cross-section (resistance) which is as uniform as possible and to increase the energy efficiency of the solar cell. At the same time, a power line having the least possible electric resistance has to be enabled by the contact fingers with respect to the energy efficiency of the solar cell, i.e. the contact fingers must have the largest possible aspect ratio since the electric resistance of the contact fingers depends on the cross-section of the contact fingers. Possible constrictions of the contact finger reduce the conductivity of the finger, thus lowering the overall efficiency of the solar cell. Hence, the aspect ratio of the contact fingers shall be uniform, in particular over the entire length of the contact fingers, if possible.
The below described drawbacks result when printing screens are used, in particular for applying the contact of the solar cell to the solar cell substrate. The screen mesh and in particular intersections of the screen mesh in the region of the printed image openings of the photoemulsion layer impair the uniformity of the application of the paste to the substrate of the solar cell during the printing process. This leads to disadvantageous constrictions in the conductor cross-section of the contact fingers and to a disadvantageous corrugated edge of the printed image, particularly resulting from screen mesh intersections closely abutting against the print edge (edge of the printed image opening). Furthermore, the maximum paste strength achievable and thus the maximum height of the printed contact fingers achievable, with respect to which the aspect ratio is directly proportional, are highly limited by the screen mesh structure in the region of the printed image openings.
Furthermore, the mesh is expanded during the printing process as a result of the contact pressure and the movement of the doctor blade on account of the resilient properties of the screen mesh, which may distort the printed image. When the substrate is printed several times using various printing screens, a printing process is usually carried out in several steps with various screens to print parts of the contact in steps. When conventional printing screens are used, asymmetries in the overall printed image can occur in transitional regions of the overall printed image, in which printed images of various printing screens border on one another, on account of the above described distortion of the individual partial printed images.
In addition, the use of printing screens requires a very fine mesh serving for stabilizing the printing screen due to the aspired large open region of the printed pattern of the screens. However, said mesh is highly susceptible to damage, thus only permitting short service lives.
With respect to the above described drawbacks regarding the use of conventionally known printing screens for printing solar cell substrates so as to apply a contact to a substrate, in particular to a substrate of a solar cell, patent application DE 10 2011 003 287 proposes a solution for applying a contact to a substrate, in particular to a substrate of a solar cell.
A printing stencil proposed according to DE 10 2011 003 287 comprises a carrier layer and a structure layer for the printing stencil, said layer being located beneath the carrier layer, wherein the structure layer has an elongate printed image opening, corresponding to at least part of the printed pattern. The elongate printed image opening in the structure layer here corresponds e.g. to the printed pattern of a contact finger of a front side contact for a solar cell. In the region of the printed image opening, the carrier layer comprises elongate carrier layer openings which extend in the longitudinal direction of the printed image opening, are substantially rectangular and/or optionally slightly rounded on the corners and, in each case, are separated from one another by a stabilizing web. The printing stencil is suited to apply to the substrate a printing medium, such as a contacting material, through at least one opening by overlaying, in a top view of the printing stencil, the carrier layer openings with the printed image opening in such a way that the printing stencil has an opening which is formed from the printed image opening and the carrier layer openings and through which the printing medium, such as a contacting material, can be applied to the substrate. The printing medium extends below the carrier layer to give a uniform 3-dimensional form.
According to a prior art method, the carrier layer openings can be made in the carrier layer by means of laser cuffing. Here, it is conventionally known to position and focus a laser beam of a laser device by means of a focusing apparatus at a position at the edge (e.g. in a corner) of a carrier layer opening to be made. In doing so, the laser beam is substantially focused directly on the surface of the carrier layer (i.e. substantially in a focal point focused directly on the surface of the carrier layer), optionally some μm above or below the surface of the carrier layer depending on the laser cuffing method. Having opened a shutter apparatus of the laser device, the focusing apparatus is controlled so as to guide the laser beam for cuffing out the carrier layer opening along the edge of the carrier layer opening to be made to cut the carrier layer opening along the edge out of the carrier layer (see e.g. FIG. 3A).
In this connection, it is necessary to guide the laser beam with very high precision when the carrier layer openings are made, and furthermore the focused laser beam must be guided once around the entire circumference of each carrier layer opening. Therefore, such a manufacturing method is very time-consuming. If the printed image opening of the structure layer is a printed image opening for a contact finger of a front contact for a solar cell, it is necessary to cut out a plurality of individual carrier layer openings for each of the numerous contact fingers, and therefore the periods for making the carrier layer openings for the production of a single printing stencil are very long and additionally require the use of very cost-intensive laser cuffing devices.